Co/C multilayers with a period thickness of 3.54 nm and 30 bilayers were deposited by direct current magnetron sputtering with different background pressures. The effects of residual background gases were investigated. The films were characterized by using grazing incidence hard x-ray reflectivity, soft x-ray reflectivity, and x-ray photoelectron spectroscopy. The results indicate that the x-ray reflectivity of Co/C multilayers decreases with increasing background pressure as well as the increasing interlayer roughness. The inclusion of more residual background air increases the interdiffusion of Co and C layers.